Staggered frequency signal distribution



Jan. 30, 1940. R Q c s 2,188,499

STAGGERED FREQUENCY SIGNAL vDISTRIBUTION Filed Dec. 21, 1937 3Sheets-Sheet 1 INTEGRATION NETWORK I I 2a COMPOSITE FILTER NETWORK l6 920 2/ 22 PuwER AMP.

FILTER MODULATOR v AMP.

AUDIO SOURCE A B c n E HARMDNIt 2 GENERATOR E T EQ I INVENTOR fiz'wfimG. c1A/2/ /i/J ATTORNEY.

Jan. 30, 1940. R. c. CURTIS STAGGERED FREQUENCY SIGN AL DISTRIBUTION 3Sheets-Sheet 2 Filed Dec. 21, 1937 Mora-um nu-15x0 IINVENTOR BY 0. CW

ATTORNEY.

Jan. 30, 1940. R. c. CURTIS STAGGERED FREQUENCY SIGNAL DISTRIBUTIONFiled Dec. 21, 1937 3 Sheets-Sheet 5 FREQUENCY DINVENTOR BY 6144 301445C.

ATTORNEY.

Patented Jan. 30, 1940 UNITED STATES STAGGERED FREQUENCY SIGNALDISTRIBUTION Richard 0. Curtis, East Orange, N. J., assignor to WiredRadio, Inc., New York, N. Y., 'a corporation of Delaware ApplicationDecember 21, 1937, Serial No. 180,971

Claims.

This invention pertains in general to signalling systems andspecifically relates to an arrangement for simultaneously transmittingand selectively receiving plural signals.

The principal object of the invention consists in providing atransmission system for generating a plurality of carrier frequenciesfor com bining with different modulation frequencies to produce pluralmodulated high frequency channels of special frequencyinter-relationships for transmission purposes.

A further object comprises providing a system and method of transmissionand reception in which a plurality of programs of special frequencyrelationships are transmitted to a receiving point, the frequencycharacteristics of the transmitted frequencies and the receivingcircuits cooperating to produce the eflicient selective reproduction ofa particular transmitted program channel.

These and other objects will be apparent from the following, referencebeing had to the accompanying drawings in which like reference numeralsdesignate corresponding parts, and in which:

Fig. 1 is a schematic representation of one embodiment of a transmissionsystem in accordance with the invention;

Fig. 2 is a diagrammatic representation of the reception circuitemployed in one embodiment of the invention; and

Fig. 3 is a graphical representation of interrelated frequencycharacteristics in accordance iwith the operation of the system of theinvenion.

The invention contemplates the production of a plurality of modulatedhigh frequency chan-' nels, the carrier frequencies and side band fre-,quencies of which are staggered along the frequency spectrum in aspecial relationship. In accordance with the invention, the frequencyseparation between any given carrier and the side band components ofadjacent carriers is essentially greater than the audio frequency cutoffof the associated receiver detector-output. Accordingly, the receiver isdesigned to perform a selection operation among the various transmittedchannels to reproduce a particular selected program. The system mayincorporate fixed carrier transmission or may utilize suppressedcarriertransmission, such different modes of operation being possible throughappropriate changes in the circuits, but still carrying out the purposeand scope of the invention.

The transmission system Referring to the drawings in detail, thetransmitter in the present embodiment of the invention comprises a basefrequency supply source I, which may be a crystal controlled electronicoscil- 5 lator or the like for supplying a standard base frequency, suchas kilocycles. The output of the base frequency source I is directed toa harmonic generator 2 which generates harmonics of the base frequency.In the present embodiment, these harmonics are delivered tomodulator-amplifiers 3-1, as frequencies of 30, 50, 60, 80, and 90kilocycles, respectively.

Sources of modulation frequencies 8l2 provide various modulationfrequencies to the modulators 3-1, individual to program channels A-E.The modulators 3| combine the respective carrier frequencies andmodulation frequencies to produce modulated high frequency energyincluding side bands of frequencies for the chan; nels A-E. Filtersl3l'l restrict the output of the modulators 3l so as to formamplitudefrequency characteristics as represented in Fig. 3. It will beseen that the program channel carriers A, B, C, D and E, respectively,derived from the modulators 3-1, are spaced in a special arrangementalong the frequency spectrum. The carrier frequencies A and B arepaired, and the carrier frequencies C and D are paired, and

so on; the carrier frequency E forming the lower component of anuncompleted pair. of carrier frequencies. The filters |3l'| operate tosubstantially eliminate all of the side band frequencies except thosealong the frequency spectrum between each pair of carrier frequencies.It 'will be noted that, as a result, a staggered frequency relationarises. Substantially no modulation side band frequencies exist betweenthe carrier frequencies BC or D-E.

Mention has heretofore been made ofthe car- 40 rier frequencies,although in the present embodiment of the invention, the carrierfrequencies A-E are eliminated or suppressed, so that high frequencyenergy in the form of side-band frequencies with carrier suppressed istransmitted over the medium 25. However, fixed carriers may be utilizedwith corresponding modifications of the system, as before pointed out. Aform of suppressed carrier system is disclosed in Edmund A. Laport,United States Patent No. 2,089,561. issued August 10, 1937.

The outputs of filters l3l'| are directed through power amplifiers I8-22 and thence through a composite filter 23 and an integration network24 to a transmission mediumii. 5

The transmission medium 215 may be of various forms, as, for example, inwired radio broadcasting, it may be a power transmission network.

The composite filter network 23, and the integration network 24,cooperate to produce the conjoint and integrated distribution of thevarious frequencies over the transmission medium. The detailedconstruction and operation of such circuits is disclosed in a copendingapplication of Henry R. Butler, Serial No. 161,389, filed August 28,1937.

The reception system In accordance with the invention, a special form ofreceiver is employed having frequency characteristics cooperating withthe special arrangement of staggeredfrequency transmission so as to makepossible an emcient reproduction of selected programs with a minimum ofinterference, and without requiring critical adjustment in the receiver.

Referring to Fig. 2, a primary selection circuit is divided into twoparts 30 and 3| The selection circuit 30 has its input connected throughcapacitive couplings with the transmission medium 25 for derivingtherefrom the carrier frequency energy depicted in Fig. 3. The circuits303| comprise an inductively coupled filter with adjustable capacitorswhich are interconnected by a common mechanical operating element 32 forbroadly selecting frequencies along the frequency spectrum depicted inFig. 3. The circuits 303| are essentially broadly tuned and have atransmission curve represented by the curve 33 in Fig. 3, this curvebeing shiftable along the frequency spectrum in accordance with themanipulation of the element 32.

A multi-electrode tube 35 is interposed between the circuits 30 and 3|.The tube 35 may be of various types suitable for the purpose, although atype 6L7 has been satisfactorily employed in the present embodiment ofthe invention. The primary control electrode of this tube is connectedwith the output of circuit 30, and the secondary control electrode ofthe tube is connected with a replacement carriers source 36. Thereplacement carriers source 36 generates carrier frequenciesrespectively equal, within two to ten cycles, to the carrier frequenciesA-E in Fig. 3, which are suppressed in transmission. The number ofcycles per second that such replacement carriers may vary from thecarriers which they replace depends upon the quality and type of programbeing received, for instance, about two cycles for high quality music orten cycles for speech. The tube 35 thus operates to superpose areplacement carrier frequency on each of the incoming side bands fromcircuit 30, with the band of frequencies selected by the circuit 30predominating over the others.

The screen electrode of tube 35 is connected with a suitable source ofpositive voltage 34 and the suppressor electrode thereof is connectedwith the cathode. The anode of tube 35 is connected with the input ofthe circuit 3 The output of circuit 3| is connected with a full wavediode detector tube 40. The detector tube 60 is substantially a lineardetector, a tube of type 6H6 having been used successfully. The pluralanodes of the tube are connected in an output circuit comprisingresistance 39 and coupled with tube 4|, which is an audio frequencyamplifier tube. The output of the tube 4| is then connected through acircuit 42 to the reproducing or utilization load.

It should be noted that the output resistance 39 of detector tube 40 isnot by-passed with a condenser. The current through resistance 39therefore follows the instantaneous values of current from circuit 3|except that, due to the rectifying action of tube 40, the current inresistance 39 is always in the same direction. The audio frequencypotential thereby developed across resistance 39 is the average of therectified carrier frequency impulses and not the peak valuecorresponding to the envelope thereof such as obtained when a by-passcondenser is employed. For the purpose of this specification, asubstantially linear detector tube, such as 40, having a substantiallynon-reactive output circuit, such as 39, is called an average-typelinear detector.

The circuit 42 limits the audio frequency output of the tube M, and, ingeneral, provides a cut-off limitation for the output of detector tubeAll. The frequency range below the cut-off frequency of the circuit 62should be made less than the effective frequency range of separation inthe staggered frequency arrangement of transmission. Assume, in Fig. 3,that the side band of carrier B at kilocycles had been selected by thecircuits 30 and 3| as represented by curve 33. The circuit 42 would thenhave an audio frequency response with respect to the detected audiofrequencies substantially as indicated by curve 15 in Fig. 3. It will beseen that this curve is limited to substantially 10 kilocycles of theaudio frequency spectrum, it being understood that the use of the curve45 in Fig. 3 is for the purpose of showing the relative cut-off of theside band frequencies after conversion to audio frequencies, as suchcut-off would be related to the high frequencies.

In the operation of the system, the frequency separation between anygiven carrier frequency of Fig. 3 and the side band components of theadjacent carriers is greater than the cut-off limitation of the detectoroutput as determined by the circuit 42. The tube 35 functions to amplifyin addition to replacing the carriers on the side band frequenciesselected by circuit 30, such superposed and amplified frequencies beingdelivered then to circuit 3|. The circuit 3| operates to furthereliminate frequencies except those within the broad curve 33.

The detector tube 40, functioning as a substantially linear detector,receives the output of tube 35 with one carrier and its side band mademore predominant by selection in circuit 3|, and performs a combiningoperation by difference or beat detection to reproduce only thefrequencies which result from the predominant carrier beating with allother frequencies within the selectivity curve 33. It is characteristicof a linear detector that it reproduces in its plate circuit only thebeat frequencies due to the predominant carrier in its input circuit,and that beats between the frequencies of adjacent side bands which maybe present in said input circuit do not appear in said plate circuit. Incontra-distinction to such characteristic of a linear detector, anexponential detector would reproduce the beats of adjacent side bandswith each other, which would appear as unintelligible audio-frequencyinterference in the output. Thus, as will be noted from Fig. 3, the sideband frequencies representing high audio frequencies associated withrelatively weak carrier A and those associated with the predominantcarrier B, respectively, are so closely adjacent each other that theirfrequency differences are in the audio range. However, the beats betweensaid side band frequencies will not appear in the output of lineardetector tube 45, Fig. 2, whereas such beats would appear if anexponential detector were used. Nevertheless, the beats of the side bandfrequencies associated with carrier A against the' strong carrier Bitself, will appear in the output of tube 40 and will be amplified bytube 4|, but they will be cut-off by circuit 42 because all such beatsare higher than ten kilocycles per second. The same applies to beatingof carrier B with the relatively weak carrier C and the portion of itsside band within the selectivity curve 33.

It will be understood that the system is not necessarily limited tosuppressed carrier and that fixed carrier and variable carrier modes ofoperation may be employed if desired. Further, various changes can bemade. For example, the source 36, in the suppressed carrier mode ofoperation, instead of supplying all of the replacement carriers to thetube 35, can be operated to supply a selected replacement carrier to becombined with the selected side band frequencies corresponding to theprogram to be reproduced.

These and other variations are obvious to those skilled in the art, butsuch variations are clearly within the intended scope and teaching ofthe invention. Therefore no limitation of the invention is intendedexcept as set forth in the claims which follow:

What is claimed as new and original to be secured by Letters Patent ofthe United States is:

1. A receiver for a group of frequencies comprising side bands thecarrier frequencies of which are spaced along the frequency spectrum sothat adjacent side bands do not overlap and so that no twocarrierfrequencies are spaced apart less than the width of one side band, saidreceiver comprising a broadly tuned selector circuit adapted to make aselected carrier predominate over the other frequencies, but notcompletely excluding other carriers or their 'side bands, a lineardetector connected to said selector circuit and having a load circuit ofsubstantially zero reactance, whereby the only beat frequencies detectedare those due to the selected carrier beating against all otherfrequencies within the range of said selector circuit, and a circuit forlimiting the detected beat frequencies in the receiver output to a rangesubstantially corresponding in width to the selected side band.

2. A receiver as defined in claim 1, having an audio frequency amplifierbetween said linear detector and said output limiting circuit, andresistance coupling between said amplifier and said detector.

3. A receiver for a group of suppressed carrier single side bands whichare spaced along the frequency spectrum so that adjacent side bands donot overlap and so that no two of their carrier frequencies are lessthan a side band apart, said receiver comprising a broadly tunedselector circuit adapted to make a selected side band predominate overthe others, means connected to said circuit for superposing replacementcarriers on the respective side bands, a second broadly tuned selectorcircuit following said means and adapted to make the selected side bandand its replaced carrier more predominant but not completely excludingother side bands or their carriers, a linear detector connected to thesecond selector circuit, and an audio frequency circuit limiting theoutput of said detector to a frequency range no wider than one said sideband.

4. A receiver comprising broadly tuned selection circuits and asubstantially linear average type detector, the breadth of tuning ofsaid circuits being sufficient to include with the selected carrier acarrier and side band portion of an adjacent channel but being adaptedto make predominant the selected carrier and its side band, whereby theonly heat frequencies detected are those due to the selected carrierbeating against all other frequencies within the range of said selectioncircuits, and means for limiting the detected beat frequencies to arange substantially corresponding in width to the side band of theselected carrier.

5. A receiver for suppressed carrier single side bands comprising afirst selection circuit for frequcncies in the vicinity of a desired oneof said side bands, means for superposing a replacement carrier thereon,a second selection circuit for the band.

- RICHARD C. CUR'I'IS.

